{
  "port": 9900,
  "protocol": "both",
  "service": "IUA — ISDN Q.921 User Adaptation",
  "oneLiner": "IUA, the SIGTRAN protocol that backhauls ISDN Q.921 signalling over IP — normally SCTP, with TCP and UDP registered too.",
  "whatItIs": "RFC 4233 defines IUA, which carries ISDN Q.921-user messages such as Q.931 call signalling between a Signaling Gateway and a Media Gateway Controller over IP. The RFC states the registered port for IUA is 9900, and IANA lists it on SCTP, TCP, and UDP; SCTP is the intended transport, so a TCP listener on 9900 is often something else entirely. You meet IUA inside telecom cores and in softswitch stacks bridging legacy ISDN to VoIP, not on general-purpose servers.",
  "exposure": {
    "verdict": "never",
    "note": "Telephony signalling between gateway elements, with no authentication of its own — it assumes a trusted operator network. Keep it on the signalling network."
  },
  "checkCommand": "sudo lsof -nP -iTCP:9900 -sTCP:LISTEN",
  "links": [
    {
      "title": "RFC 4233 — ISDN Q.921-User Adaptation Layer",
      "url": "https://www.rfc-editor.org/rfc/rfc4233.html",
      "kind": "spec"
    },
    {
      "title": "IANA Service Name and Transport Protocol Port Number Registry",
      "url": "https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml",
      "kind": "iana"
    }
  ]
}